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Odour Impact Assessment

An Odour Impact Assessment is a report that looks to gauge impacts from a proposed development adjacent to an existing receptor (new sewerage works next to house), or impacts on a proposal from an existing source (new homes next to sewerage works). Very occasionally both receptor and source might be new, and we are assessing the odour "relationship" between them.

Southwest Environmental Limited can undertake Odour Impact Assessment for you Planning Application or Permit Application. We use modern computer modeling methods, with atmospheric data to provide reports on a quick turn around.


Click on below links to skip to various topics:

Odour Impact from Various Sources

Planning Appeals Involving Odour

Odour Sniff Testing

Odour Impact Assessment Methods

Objecting Against a Scheme on the Grounds of Odour Impact

Weather Data for Odour Assessment

Odour Mitigation (Reduction)

Acceptable Levels of Odour

Odour Impact Assessment Methods

Smell is a very subjective thing, the nicest of smells such as baked bread or roses, would hardly be considered a problem, however the worst of smells such as rotten flesh, or rotten eggs are a real burden on the nostrils. In order to assess odour a unit has been developed, an Odour Unit (ou), 1 ou/m3 is the limit at which humans can smell an odour, dependent on its character (hedonic score) am upper limit of acceptability is then set. For example livestock (chickens or pigs) have a limit set at 3 ou/m3, whilst something more unpleasant such as an abattoir would have an odour limit set at 1.5 ou/m3. The odour limit for AD Plants (Anaerobic Digestion) is set at 3 ou/m3.

Odour Modelling

odour model mapUsing a computer model (or a calculator if you are particularly patient!), you can work out odour levels (concentrations) at set points away from the source (chicken shed or factory), these data points can then be used to create contour, and so gauge odour concentration at a particular point away from the source.

The calculation usually employed by computer models is the "gaussian distribution" this is named after the famous mathematician Carl Friedrich Gauss, who was an inspired mathematician even as a young child. This distribution model is also used in Ammonia Emissions Assessments.

The image of the sewage works (black and white) shows Salisbury Sewage Works. Learn more about the Layout of Salisbury Sewage Works

If you are looking to locate a Sewage Works you can use our handy Sewage Works Location Maps to do so.

Odour Plumes

odour_impact_assessmentFrom this we end up with a modeled odour plume, which can be used for the basis of an odour impact assessment.

Of course these are modeled results, and all their aim to represent real world conditions, they will not.

Even the most complex models available and relatively simplistic in the method they use to determine dispersion of odour plumes. More complex models will use actual weather data taken from the study area to deuce likely conditions, presenting concentrations as a probabilistic concentration rather than a worst case scenario. This method is "above board" but is likely to under represent concentrations.

With sensible siting and implementation of odour management plans specific to the activity on site, odour concentration can be kept below thresholds, based on worst case scenarios.

Odour Impact from Various Sources

odour assessment plume

A model not real. You place numbers in to the model, press "go" and it gives you another number. The model developers have done there best to provide an accurate representation of real world scenarios.

There are some interesting factors that are observable when using the model. In that "area" type sources such as waste heaps or farm yards typically fair worse than "point" sources such as chimney or extract vents.

By example a farm yard which is covered in a hypothetical manure will fair worse than a cattle shed of the same area with point source ridge fans. This is strange, and unlikely to represent real world  scenarios, but the model is the best method at present so what ever the out put may be it is usually accepted.

Odour Mitigation (Reduction)

If it is shown that odour levels are too high then mitigative measures can be applied. This can include a variety of solutions which we recommend depending on the situation. For example a proposed slurry store on some design drawings, could be moved away a sensitive receptor such as a dwelling, to mitigate against impacts. However, an existing slurry store is not easy to move, in this instance a cover or croffles may be recommended to reduce exposed surface area.

Objecting Against a Scheme on the Grounds of Odour Impact

Most consultants use the same method for calculating odour. If the applicant has submitted an odour impact assessment with their application that points to favorable conditions at nearby receptors it is likely that our report would indicate the same. A typical method is to use a 95% or 99% certainty value concentration, this provide an "average" across the year depending on the wind direction and temperature etc.

The problem with this method is that if on occasion the odour level which be much higher than reported. If we consider just 10 days worth of data where the wind was blowing away from the receptor for 8 days . . . . we might have source odour levels of 20 ou/m3, but the reported figure would be perhaps 4 ou/m3 because of the "averaging". Now for 8 of those days the receptor location (house or school) would receive background levels of odour but for the 2 days down wind life might be unpleasant. However, this is how odour is assessed at present.

Even in cases where we have prepared reports for residents groups or parish councils against developments, they are not normally considered.  These political consideration are more important than the report itself, you may need the ear of your local councilors in order to make any adversary effective.

Planning Appeals Involving Odour

Below is a list of Planning Appeals were odour was a key factor in the determination of the appeal. You can fin out more about each case by visiting the Planning Inspectorate and search the last 7 digits of each appeal reference number.

Acceptable Levels of Odour

Typically a Odour Level of between 1.5 and 3.0 EOm3 is deemed acceptable. However for nastier (more offensive) smells this 3.0 EOm3 value can be very close to the upper limit of what is acceptable. Residents adjacent to a Sewage Treatment works in Gillingham ( APP/N1215/W/15/3005513 )complained about odour, and later modeling put their property at around 3.0 EOUm3.

Most of the objections that  referred to odours and the sewage works came from residents in the streets  nearest to the site. As was demonstrated in cross-examination of residents  who spoke at the Inquiry, these lie within, or close to, the 3 OUE/m3 contour  line which is suggested by the appellant as the threshold for acceptable  location of any housing within the site. And, although they are demonstrative  of an experience of odour, it is an experience which has been tolerated, in  some cases, for nearly twenty years without leading to a move away from the  area or to a greater degree of complaint than when the opportunity offered  itself.

 Taking all the above into account, the planning inspector concluded that the appropriate parameter to  apply in this case is the 3 OUE/m3 contour line; a more restrictive approach would preclude from development areas which are comparable in odour terms  with extensive areas of existing housing. A less restrictive  approach would permit development of areas which, in odour terms, clearly  ought not to be developed. Taking all the above into account, I conclude that the appropriate parameter to  apply in this case is the 3 OUE/m3 contour line; a more restrictive approach.

Odour Assessment at Sewage Treatment Works

 there is typically less of Odour from Sewage Treatment Works is typically less of a problem than people might first think. It is true that if you are building something (house for example) next to a very large works, you will probably find that thresholds are broken, but with smaller works.

 sewage works odour

In modeling a sewage works, it is important to include all of the potential odour sources of significance. The worst offenders are the primary inlet tanks and the sludge tanks, which are normally replaced by sealed tanks in modern works. For A Trip Around a Sewage Works < Please follow this link.

Odour Sniff Testing

An assessment of uncertainty can also be included in the odour modeling report.  The reasoning here is that the accuracy of the odour modeling should be backed up with another form of assessment. This is a confusing concept as the whole point of modeling odour, it to account for its temporal nature. And here we are asked to walk around in a field sniffing, in order to prove or model is accurate.

This is a flawed concept, but we are sometimes asked to conduct sniff tests. We might understand why it is useful to undertake a sniff test as a standalone assessment, however to verify finding of a complex computer model using a sniff test is akin to poking a space shuttle with a stick and deeming fit to fly.

The IAQM gives instructions on how to conduct an odou sniff test in its excellent guide on odour assessment. Other consultees such as Wessex Water have more stringent requirements for sniff testing, and that assessors have "standard" noses under various accredited schemes.

Examples of Odour Impact Assessments